Hi-performance roof insulation?

[d000hg]

I've been looking into available products for roof insulation and all the regular ones are seemingly in the 0.02W/mK range. I was excited to come across Optim-R at .007 but am I correct thinking these are fragile vacuum units i.e. a nail through will ruin it?

Our roof design is solid boarded 18mm spruce, for typical garden use they simply supply shingles which are nailed on.
If you want to insulate they recommend a hot-roof design for condensation reasons. Up to 50mm insulation simply nail the shingles onto the board using 60mm clout nails, if you need more than that (which you surely will since the BR minimum roof value is .2W/m2 implying 100mm insulation) then the suggestion is to lay the insulation, build a frame on top using OSB or similar, add a DPM then nail shingles onto this outer frame (or something like that I'm regurgitating what I remember a builder saying). Both these options are fairly commonly used on these cabins, apparently.

So I wondered if there are any materials out there which are ultra-performing but rigid foam type construction suitable for this purpose? I am guessing not, at least at any affordable price?

I had also considered a hybrid approach - 50mm external insulation then some between the purlins as well. Purlins are approx 145 deep so quite a bit of space. We'd prefer to keep the inside open for cosmetic reasons, not sure which compromise is preferable.

Guy doing it has experience as a roofer and carpenter, seems to prefer the hot-roof option with lots of insulation outside. Does that seem reasonable?

 

[Notch7]

hybrid roof options arent tested by the insulation manufacturers -because there are so many construction options and all are difficult to ensure the vapour barrier is continuous.

on that basis, if you can, just go for a warm roof with all the insulation outside.

current building regs for an extension is 0.18 u value which is 120mm insulation,

there are arent any higher performing types of insulation for a warm roof.....its PIR or PIR -you can choose your brand though, celetex, ecotherm, kingspan etc etc.

 

[d000hg]

I'm told for our building classed as "other" (!) it's .2 by the SBEM guy but close enough.

At that thickness they recommend building a wooden frame to hold everything in place so you have rafters, board, insulation, board, shingles like a choc-ice almost. Would I then want a DPC at all outside the insulation or would that serve no purpose? It is needed when there's an abrupt temperature boundary causing condensation, which shouldn't happen?

I did see you can buy clout nails up to 150mm. Surely you couldn't nail shingles directly through 125mm of PIR - or can you? I have actually never seen PIR in person as I always lived in houses with glass-fibre insulation, I don't know how hard and strong it is.

 

[IT Minion]

PIR can take a lot of weight but it doesn't like point loads. If you bang it on a corner of something it'll dent but it is capable of being put below concrete slabs and having stupid amounts of weight sitting on it.

Most PIR comes with an aluminium foil costing which acts as a vapour barrier. You should tape the seams with aluminium tape to avoid air seeping through the gaps.

If you nail through the PIR then you're bridging the thermal layer, which isn't ideal. I believe the preferred practice is to build a framework on top of the PIR for everything else to fix to.

 

[d000hg]

I was wondering about that, putting a conductor as a bridge through the isolator. I have no idea if the area of conductance is negligible. You can put insulation between your rafters which leads to gaps and bridges on the rafters themselves, I wonder how that compares to nice unbroken layer of insulation with some nails?

I'd agree an external frame is better but we're more about what's acceptable.